Embodiments of the present invention relate to a liquid crystal display device and a phase compensation method for the same.
With their small volume, light weight, low power consumption, no radiation, good display effect and other advantages, liquid crystal display devices have got rapid development in recent years, and among them, a Twisted Nematic (TN) mode is most mature. However, a liquid crystal display device of the TN mode has its own defects. When light rays emitted from a backlight source pass through a liquid crystal display panel, the intensity of light transmitted by the display varies due to difference in the angle of the incident light rays at different viewing angles, and the display effect brought about by it also varies.
Because the viewing angle of a liquid crystal display device results from the optical anisotropy caused by the anisotropy of liquid crystal molecules fundamentally, the viewing angle characteristic of the LCD may be improved so long as a thin film with optical anisotropy is added to a surface of the liquid crystal display device to reduce a light-leakage phenomenon upon display in a dark state. This method is called as an optical compensation film mode of liquid crystal display devices. This method is considered as a simple, effective and lower-cost method because it does not change the process flow of production in prior art and has been applied widely.
However, upon display of the liquid crystal display device in a dark state, there may be two kinds of light leakage: light leakage caused by a positive liquid crystal layer of a liquid crystal cell in the liquid crystal display device and light leakage caused by a polarizer. The light leakage caused by the positive liquid crystal layer of the liquid crystal cell is usually compensated by way of superposing a negative optical film that is symmetrical to the liquid crystal layer in prior art, and for more details, please refer to FIG. 1, which shows a structure of the polarizer that is commonly used in liquid crystal display devices. In FIG. 1, the polarizer is mainly divided into two parts: a polarizing plate 10 and an optical retardation thin film 20. As for a specific structure of the polarizing plate 10, there are a protective film, a low reflective layer, an antiglare layer, a supporting layer of cellulose tri-acetate (TAC), a polarizing layer of polyvinyl alcohol (PVA), and a supporting layer of TAC in sequence from top to bottom, where, what serves a polarizing function chiefly is the polarizing layer of PVA; the chief function of the optical retardation thin film 20 is to compensate light leakage caused by the liquid crystal cell, and its material may be a double-optical-axis optical layer, and may also be a plate with negative optical anisotropy, for example, C-plate or a negative, non-toxic and liquid-crystal optical compensation layer (WV DLC). Light leakage of liquid crystals is compensated by the optical retardation thin film 20, and if the birefringence effect of the positive liquid crystal cell is to be compensated fully, then the retardation effect of the optical retardation thin film 20 has to coincide with the positive liquid crystal cell, namely,(ne−no)cdc+(ne−no)LCdLC=0where ne and no are refractivities of the optical retardation thin film to an e light and an o light, respectively, dc is thickness of the optical retardation thin film, and dLC is thickness of the liquid crystal cell. Here, a linearly polarized light is vertically incident onto a wafer, such as the optical retardation thin film or the liquid crystal cell, so that the incident light is decomposed with oscillation into two components, i.e. an o light perpendicular to an optical axis (an o oscillation) and an e light parallel to the optical axis (an e oscillation). The o light and the e light propagate along the same direction, but have different propagating speeds (owing to different refractivities), so that an optical path difference of (ne−no)d is generated between the two kinds of light after they passing through the wafer, where d is thickness of the wafer, and, ne and no are refractivities of the wafer to an e light and an o light, respectively. The e light and the o light are generally combined into an elliptically polarized light.
Although light leakage caused by the positive liquid crystal cell can be eliminated through addition of the optical retardation thin film 20, light leakage may be caused by the polarizer itself as well. When the light ray is deflected from the vertical direction, due to a polarizing function of the polarizer, the fast-axis light and the slow-axis light coexist upon it enters the liquid crystal layer, and the phase difference between them is directly proportional to the optical path difference Δnd. In this case, the phase difference makes a linearly polarized light that is obliquely incident turn into an elliptically polarized light after it passes through the birefringent liquid crystal layer, and it cannot be absorbed by the polarizer on the other side completely after it exits. Thus, a light leakage phenomenon occurs, leading to problems that the contrast of display units is degraded, the viewing angle is narrowed, the display effect becomes worse, and so on. In the state of some large viewing angles, a situation that transmittance in a dark state is greater than transmittance in a bright state might also arise, namely inversion of the contrast, which is a problem in viewing angle caused by the anisotropy of liquid crystal molecules. Therefore, light leakage caused by the polarizer cannot be ignored.